Problem: The scale on a map is 3cm : 6km. If two cities are 15cm apart on the map, what is the actual distance in km between the two cities?
Answer: The scale means that every 3cm on the map corresponds to an actual distance of 6km. A distance of 15cm on the map is the same as 5 $\cdot$ 3cm. The actual distance between the two cities is 5 $\cdot$ 6km, or 30km.